This invention relates to cermets, and more particularly relates to composite cermets having one or more layers around central core, and also relates to a process for producing them and to a lamp incorporating them.
Development of a new higher-temperature metal halide lamp requires the substitution of a ceramic arc tube, often polycrystalline alumina (PCA), for the usual fused silica arc tube, which cannot withstand these higher tempertures. Closure for the PCA tubes must have thermal expansion characteristics similar to Al.sub.2 O.sub.3, while at the same time providing a method for electrical connection to the lamp electrode. Thus, the closure material would ideally have PCA-like thermal expansion characteristics but metal-like electrical conductivity. A further complicating factor is that the closure material must also be chemically resistant to the metal halide environment. This would eliminate Nb as the closure material even though it could satisfy both the thermal expansion and conductivity requirements, as Nb reacts rapidly with the halogens.
The problem of chemical resistance is circumvented by choosing closure materials which are already acceptable as other lamp components: Al.sub.2 O.sub.3, the arc tube material, and W or Mo, the electrode and feed-through materials. A simple mixture of Al.sub.2 O.sub.3 with a small amount of W or Mo will not suffice, as approximately 20 volume percent (55 weight percent) of W, for example, is required to achieve acceptable conductivity. a ceramic/metal mixture (cermet) of such high metal content (see, e.g., U.S. Pat. No. 4,001,625) will not exhibit ceramic thermal expansion behavior, however, and problems in achieving a leak-proof closure will likely occur. On the other hand, use of lower percentages of metal will yield materials of unacceptably low conductivity; if a continuous metal feed-through which extends through the closure is used, expansion mismatch causes problems with leaks around the feed-through.
A more complex approach is represented by U.S. Pat. Nos. 4,155,757; 4,155,758 and 4,354,964. In the '964 patent, for example, a refractory metal-oxide cermet with an unusually high conductivity is achieved by forming a continuous conductive network of the refractory metal surrounding the oxide granules. This is achieved by coating relatively coarse refractory metal oxide granules with fine metal powder, compacting and sintering the powder compact. As will be appreciated, producing such a cermet requires close control over process parameters, especially particle sizes of the constituents.
It is accordingly an object of the invention to provide refractory metal-aluminum oxide cermets having both relatively high electrical conductivities and low thermal expansion coefficients, which cermets may be used as end closures for alumina arc tubes in metal halide lamps.
It is a further object of the invention to provide composite cermets having a combination of physical, chemical and electrical properties which are unattainable in one or more of the constituent materials, which cermets may be readily produced by compacting and sintering of powder constituents.